In light cured applications utilizing Type II photoinitiators (hydrogen abstraction), low molecular weight amines are typically incorporated as a coinitiator. These small molecule amines are not always fully reacted and can remain in the final cured matrix, which pose complications such as residual extractables and leachables.
Using light to cure coatings comes with motivations such as environmental compliance, fast cure, improved physical properties and lower applied cost. These motivators translate into benefits of reduced solvent emissions, increased product speed/productivity, product performance, efficiency and cost effectiveness. The use of UV-EB has enjoyed a growth rate of approximately 10% per annum over the last decade and equates to an annual industrial usage of about 100,000,000 lbs. Nevertheless, some obstacles include cost of products, equipment cost, poor weatherability, adhesion and curing of thick samples and residual uncured materials. While many of these issues are successfully being addressed, unsolved problems and deterrents still exist.
A UV curable formulation can contain several fundamental components, of which can be monomers, functionalized oligomers, and photoinitiators (free-radical or cationic). Among additional components which can also be included are, for example, pigments, dyes, light stabilizers, radical scavengers and adhesion promoters.
Free-radical photoinitiators are typed into two classes: Type I, those that undergo photocleavage to yield free-radicals and Type II, those that produce initiating radicals through an abstraction process. Type I photoinitiators produce radicals through a unimolecular fragmentation. Examples of these include aromatic carbonyl compounds, such as derivatives of benzoin, benzilketal and acetophenone. One example is 2,2-dimethoxy-2-phenylacetophenone (DMPA), the reaction pathways of which are as follows:

Upon irradiation DMPA cleavage occurs by generating the benzyl radical and a dimethoxy substituted carbon centered radical. The dimethoxy radical rearranges to form a methyl radical and methylbenzoate. The benzyl radical can initiate polymerization or abstract hydrogens forming benzaldehyde. Unreacted residual DMPA and other small molecules remain in the final polymer matrix and can be readily extracted and leached. This makes these coatings unfit for applications that involve contact with food, notably an emerging application. Residual DMPA can also further react leading to premature degradation of the polymer. These materials then cannot be used for outdoor applications where exposure to intense UV would be expected.
Thus while Type I photoinitiators typically provide high rates of initiation, yielding rapid controllable rates of polymerization and fast curing line speeds, Type I systems are often expensive and can produce toxic by-products.
Type II (abstraction type) photoinitiators are typically aromatic ketones, such as thioxanthone and benzophenone derivatives. In these systems, a coinitiator must be present in order to produce an initiating radical. These coinitiators can include amines, alcohols or ethers. The process of producing radicals is either through a hydrogen abstraction or an electron transfer mechanism depending on the coinitiator. The primary initiating radical is usually a radical centered on the coinitiator. In the presence of abstractable hydrogens (such as amine, ether, thiol or alcohol) the reaction produces two radicals. The reaction pathway may be depicted as follows:

When the hydrogen donor source is an amine, the excited state benzophenone participates in an electron transfer process forming the radical-anion/radical-cation pair. This is subsequently followed by a rapid proton-transfer from a carbon alpha to the nitrogen on the amine (aminyl radical) to the benzophenone radical-anion producing the semipinacol ketyl type radical and a carbon centered radical on the amine. The semipinacol ketyl type radical is not efficient at initiating polymerization, whereas the aminyl radical readily initiates polymerization. The products from the semipinacol ketyl type radical are still photoactive and can lead to photosensitivity of the final film.
In light cured applications utilizing Type II photoinitiators (hydrogen abstraction), tertiary amines are typically incorporated as a coinitiator, due to their reactivity with type II photoinitiators. Amine synergists can be placed in three categories: amine acrylates, amine acrylate adducts, and free-amines (ethanolamines).
Amine acrylates are made by the reaction of an amine and a multifunctional acrylate in such a ratio as to produce an oligomeric compound. Amine acrylates do not blush or discolor sensitive pigments, but are costly, can cause skin burns, have high viscosity, and must be used in the range of 12-20 wt % to be effective.
Amine acrylate adducts are formed from reacting secondary amines with an acrylate monomer. The amount of amine functionality is controlled by the ratio of amine to acrylate. The amine acrylate adducts give good cure, do not blush and contribute to overcoming the effects of oxygen inhibition, but discolor sensitive pigments, can cause skin burns, and must be used in the range of 8-12 wt % to be effective.
Free-amines (ethanolamines), which are low in cost, effective in the range of 4-6 wt %, and give good through cure as well as contribute to overcoming the effects of oxygen inhibition. However, these blush in high humidity, discolor sensitive pigments such as rhodamine red and reflex blue in over print varnishes, are extractable, and contribute to odors due to high vapor pressures. Examples of low molecular weight amines typically used as a coinitiators are N-methyl-N,N-diethanolamine, triethanolamine, triethylamine, triisopropanolamine, and N-methyldibutylamine. These small molecule amines are not always fully reacted and can remain in the final cured matrix, which pose complications such as residual extractables and leachables.
Accordingly, when forming films using Type II photoinitiators, especially thin films of about 2 mils or less, a need exists for a way of eliminating or minimizing (i) extractables in the films, (ii) discoloration of the films, and (iii) premature degradation of the films, while at the same time achieving rapid cures. It would be especially advantageous if not only films but other articles as well could be fabricated by photopolymerization using Type I or Type II initiators, or both, without increasing extractables, blushing, discoloration or degradation of the resultant article or product.